This invention relates to a stable, non-film-forming dispersion of particles in a liquid, non-polar solvent, preferably a fluorinated solvent, that includes a dispersant selected from the group consisting of highly fluorinated polyethers, having an atomic ratio of carbon to oxygen of between 2 and 4 inclusive, and bearing at least one polar functional group.
U.S. Pat. No. 5,397,669 (Minnesota Mining and Manufacturing) discloses liquid toners for use with perfluorinated solvents. The patent discloses that the compositions are film-forming, allowing them to function properly as toners. (""669 at p. 8 Ins. 3-5). The ""669 patent discloses pigment particles bound to a polymer that is highly fluorinated in specific parts, and that includes monomer units having groups that bind polyvalent metal ions. The ""669 patent also discloses pigment particles bound to a polymer that is highly fluorinated in its entirety, without requiring monomers having groups that bind polyvalent metal ions.
U.S. Pat. No. 5,530,053 (Minnesota Mining and Manufacturing) also discloses liquid toners for use with perfluorinated solvents. The toners of ""053 are polymeric dyes that are highly fluorinated in specified parts and have attached chromophoric groups. The ""053 patent discloses that the toner can form a latex in perfluorinated solvent, where the toner takes a core-shell form with the hydrocarbon portion in the core and the fluorocarbon portion in the shell.
U.S. Pat. No. 5,919,293 (Hewlett-Packard) discloses ink jet inks composed of colorants in Fluorinert(trademark) solvents (Minnesota Mining and Manufacturing Co., St. Paul, Minn.), which are perfluorinated or nearly-perfluorinated alkanes.
U.S. Pat. No. 5,573,711 (Copytele) discloses the use of certain polymeric fluorosurfactants in electrophoretic image displays. The ""711 patent teaches the use of Fluorad(trademark) surfactants (Minnesota Mining and Manufacturing Co., St. Paul, Minn.), including FC-171, having the structure Rfxe2x80x94SO2N(C2H5)(CH3CH3O)nCH3, where n is about 8 and Rf is a fluorocarbon portion.
U.S. Pat. No. 4,356,291 (Du Pont) discloses hexafluoropropylene oxide polymers terminating in a variety of end groups and methods of making same.
Co-assigned patent applications U.S. Ser. No. 09/604,894, 09/604,889 and 09/605,211 are currently pending. The disclosures of those applications are incorporated herein by reference.
Briefly, the present invention provides a stable, non-film-forming dispersion comprising a) dispersed particles, b) a liquid, non-polar solvent, and c) a dispersant selected from the group consisting of highly fluorinated polyethers, having an atomic ratio of carbon to oxygen of between 2 and 4 inclusive, and bearing at least one polar functional group which is preferably an ionizable group such as a carboxylic acid group.
What has not been described in the art, and is provided by the present invention, is the highly stable dispersion achieved by use of the dispersant according to the present invention which is particularly useful in an electrophoretic display device.
In this application:
xe2x80x9chighly fluorinatedxe2x80x9d, means containing fluorine in an amount of 40 wt % or more, but preferably 50 wt % or more and more preferably 60 wt % or more, and refers to the fluorine content of a population of chemical moieties where applicable, such as in the term, xe2x80x9cone or more highly fluorinated macromersxe2x80x9d;
xe2x80x9cnon-fluorinatedxe2x80x9d, means containing substantially no fluorine, i.e. containing fluorine in an amount of 5 wt % or less, but preferably 1 wt % or less and most preferably 0 wt %, and refers to the fluorine content of a population of chemical moieties where applicable, such as in the term, xe2x80x9cone or more non-fluorinated free-radically-polymerizable monomersxe2x80x9d;
xe2x80x9cionizable functional groupxe2x80x9d, means a functional group that may ionize in water, such as carboxyl groups, acidic sulfur-containing groups such as xe2x80x94SO3H and xe2x80x94SO2H, acidic phosphorus-containing groups such as xe2x80x94PO3H2, and the like;
xe2x80x9chydrogen bonding functional groupxe2x80x9d, means a functional group having a hydrogen atom available for hydrogen bonding, such as functional groups containing xe2x80x94OH, xe2x80x94NH or xe2x80x94SH moieties, including hydroxyl groups, amino groups, and the like;
xe2x80x9cpolar functional groupxe2x80x9d, means an ionizable functional group or a hydrogen bonding functional group, preferably being an ionizable functional group and most preferably being carboxyl;
xe2x80x9cC(number)xe2x80x9d refers to a chemical moiety containing the indicated number of carbon atoms;
xe2x80x9c(meth)acrylatexe2x80x9d means acrylate and methacrylate; and
xe2x80x9csubstitutedxe2x80x9d means, for a chemical species, substituted by conventional substituents that do not interfere with the desired product or process, e.g., substituents can be alkyl, alkoxy, aryl, phenyl, halo (F, Cl, Br, I), cyano, etc.
It is an advantage of the present invention to provide a highly stable dispersion of pigment or dyed latex particles that is particularly useful in an electrophoretic display device.
The present invention provides a stable, non-film-forming dispersion comprising a) dispersed particles, b) a liquid, non-polar solvent, and c) a dispersant selected from the group consisting of highly fluorinated polyethers, having an atomic ratio of carbon to oxygen of between 2 and 4 inclusive, and bearing at least one polar functional group which is preferably an ionizable group such as a carboxylic acid group.
The dispersed particles according to the present invention may be any suitable particles. Preferably the particles have an average diameter of 1000 nm or less, more preferably 350 nm or less, more preferably 300 nm or less, more preferably 250 nm or less, and most preferably 200 nm or less. Preferably the particles are organic. Preferably the particles are non-fluorinated. In one preferred embodiment, the dispersed particles are particles of pigment material. Preferably the pigment particles consist of pigment crystalite aggregates. These aggregates are preferably encapsulated, either partially or fully, by dispersant that is anchored or adsorbed to the particle surface. In another preferred embodiment, the dispersed particles are latex particles, more preferably (meth)acrylic latex particles. The latex particles may contain dyes that may be dispersed in the latex particles or covalently bound. The latex particles may be homogeneous or may have a core-shell structure. The latex particles are preferably encapsulated, either partially or fully, by dispersant that is anchored or adsorbed to the particle surface.
The solvent may be any suitable non-polar solvent that is liquid at room temperature. Preferably, the solvent is a hydrocarbon solvent, more preferably a fluorinated hydrocarbon solvent. More preferably, the solvent is a highly fluorinated solvent, especially a branched or unbranched, cyclic or non-cyclic fluoroalkane. Most preferably the solvent is perfluorinated. Preferred solvents include FLUORINERT(trademark) fluorinated solvents available from 3M Company, St. Paul, Minn. Two especially preferred solvents are FLUORINERT FC-75, a perfluorinated C8 solvent, CAS No. 86508-42-1, and FLUORINERT FC-84, a perfluorinated C7 solvent, CAS No. 86508-42-1. The solvent preferably has a dielectric constant of 3.0 or less.
The density of particles in solvent (solids content) may be any level at which the dispersion is stable and does not significantly coagulate. For use of the dispersion in an electrophoretic display, the solids content may be any level that allows proper functioning over repeated cycles. Preferably, the solids content is less than 10 wt %, more preferably less than 5 wt %, and most preferably less than 2 wt %.
The dispersant is a highly fluorinated polyether having an atomic ratio of carbon to oxygen of between 2 and 4 inclusive, preferably about 3, and bearing at least one polar functional group. Preferably the dispersant is perfluorinated in its backbone. Preferred dispersants are described by Formula I:
nxe2x80x94C3F7Oxe2x80x94(CF(CF3)xe2x80x94CF2O)nxe2x80x94CF(CF3)xe2x80x94COOHxe2x80x83xe2x80x83(I)
Most preferably the dispersant is perfluorinated throughout. Preferred dispersants comprise poly(hexafluoropropylene oxide) polymers, poly(tetrafluoroethylene oxide) polymers and poly(octafluorobutylene oxide) polymers, and polymers containing various combinations of (hexafluoropropylene oxide), (tetrafluoroethylene oxide) and (octafluorobutylene oxide) units. Most preferably the dispersant comprises a poly(hexafluoropropylene oxide) polymer. The molecular weight of the dispersant is preferably in the range of 2,500-7,500.
The polar functional group may be either an ionizable functional group or a hydrogen bonding functional group, as defined above, but is preferably an ionizable functional group, more preferably an acidic group, and most preferably a carboxyl group. Preferably the polar functional group is a terminal or end group or the dispersant.
The dispersions according to the present invention may be incorporated into electrophoretic displays. A typical display comprises two planar electrodes defining a thin gap between them which holds the dispersion. When a sufficient voltage of the correct polarity is applied, the dispersed particles are drawn out of suspension and onto one electrode. That electrode, which is substantially transparent, forms the inner surface of a viewing glass, such that the particles form an image viewed through the glass. In contradiction to the characteristics of an electrostatic toner, which must form a permanent image under analogous conditions, the dispersed particles of the present invention must return to dispersion when the voltage is removed or reversed.
The dispersions according to the present invention have high resistance to film formation when used in electrophoretic display devices. To determine resistance to film formation, an actual device may be used or a breadboard device as described in the examples below. Dispersions of any solids content may be tested but preferably the solids content is 1 wt %. The device is preferably used in a normal manner, alternately applying and removing (or reversing) the typical use voltage. The voltage should be sufficient to remove particles from suspension and create an image when applied. Preferably the dispersions are non-film-forming to the extent that they redisperse completely (by appearance to the naked eye) after at least twenty cycles, more preferably after at least 100 cycles, and most preferably after at least 10,000 cycles.
The dispersion and/or the dispersed particles may also include charging agents. The charging agent renders the particle mobile under the influence of an electric field. In addition, the charge imparted to the particles by the charging agent creates an electrostatic repulsion between particles which improves resistance to film formation. The charging agent is preferably cationic, more preferably a quaternary ammonium cation. Preferred charging agents include 1-ethyl-3-methyl-1H-imidazolium bis(trifluoromethylsulfonylamide), which may be prepared as disclosed in the examples below; (C4H9)3N:HOC(O)xe2x80x94C7F15; (C3H7)4N+xe2x88x92OC(O)xe2x80x94C7F15; (C4H9)4N+xe2x88x92OC(O)xe2x80x94C9F19; C7F15xe2x80x94CO2H; and combinations thereof.
In electrophoretic display applications, dispersions according to the present invention preferably demonstrate a high conductance as measured by the method described in the examples below. Measured conductance is taken to reflect the charge/mass ratio (charge density) of the particles in suspension, whether imparted by the charging agent or inherent in the particle itself. Preferred dispersions according to the present invention have a conductance of 1 picomho/cm or more, more preferably 5 picomho/cm or more, more preferably 40 picomho/cm or more, and most preferably 80 picomho/cm or more.
Preferably the dispersion according to the present invention has a high conductance without the addition of a charging agent; preferably a conductance of 1 picomho/cm or more, more preferably 5 picomho/cm or more, more preferably 40 picomho/cm or more, and most preferably 80 picomho/cm or more.
This invention is useful in electrophoretic display devices.